Isothiosemicarbazones, except N2-substituted ones, were found to exist exclusively in the amino structure by means of i.r. and n.m.r. spectroscopy. The cis configuration (N1cis to N4 about the N2=C bond) with intramolecular hydrogen bonding was proposed as the predominant structure for N4-unsubstituted isothiosemicarbazones on the basis of the i.r. spectra from the partially deuterated compounds. The trans configuration (N1trans to N4 about the N2=C bond) was identified in all N4-monsubstituted and in a few N4-unsubstituted isothiosemicarbazones. The cis/trans ratios for the former varied depending on the solvent used.